Transgenerational genomic instability as revealed by a somatic mutation assay using the medaka fish

Seasonal variations in photoperiod affect hepatic metabolism of medaka (Oryzias latipes)

Organisms living in temperate regions are sensitive to seasonal variations in the environment; they are known to accumulate energy as fat in their livers during the winter when days are shorter, temperatures are lower, and food is scarce. However, the effect of variations in photoperiod alone on hepatic lipid metabolism has not been well studied. Therefore, in this study, we analyzed lipid metabolism in the liver of medaka, Oryzias latipes, while varying the length of days at constant temperature. Larger amounts of fatty acids accumulated in the liver after 14 days under short‐day conditions than under long‐day conditions. Metabolome analysis showed no accumulation of long‐chain unsaturated fatty acids, but showed a significant accumulation of long‐chain saturated fatty acids. Short‐day conditions induced a reduction in the levels of succinate, fumarate, and malate in the tricarboxylic acid cycle, decreased expression of PPARα, and decreased accumulation of acylcarnitine, which suggested inhibition of lipolysis. In addition, transparent medaka fed on a high‐fat diet under short‐day conditions exhibited greater amounts of fat accumulation and developed fatty liver. The findings of our study will be useful for creating a medaka hepatic steatosis model for future studies of hepatic steatosis‐related diseases.

The sine qua non of the fish invitrome today and tomorrow in environmental radiobiology

Fish cell lines, collectively referred to as the fish invitrome, are useful diagnostic tools to study radiation impacts on aquatic health and elucidate radiation mechanisms in fish. This paper will highlight the advantages, discuss the challenges, and propose possible future directions for uses of the fish invitrome in the field of environmental radiobiology. The fish invitrome contains at least 714 fish cell lines. However, only a few of these cell lines have been used to study radiation biology in fish and they represent only 10 fish species. The fish invitrome is clearly not yet explored for its full potential in radiation biology. Evidence suggests that they are useful and, in some cases, irreplaceable in making underlying theories and fundamental concepts in radiation responses in fish. The debate of whether environmental radiation is harmful, presents risks, has no effect on health, or is beneficial is on-going and is one that fish cell lines can help address in a time-effective fashion. Any information obtained with fish cell lines is useful in the framework of environment radiation risk assessments. Radiation threats to aquatic health will continue due to the very likely rise of nuclear energy and medicine in the future. The fish invitrome, in theory, lives forever and can meet new challenges at any given time to provide diagnostic risk analyses pertaining to aquatic health and environmental radiation protection.

In Vivo Analysis of Embryo Development and Behavioral Response of Medaka Fish under Static Magnetic Field Exposures

The static magnetic field (SMF) in human exposure has become a health risk concern, especially with respect to prolonged exposure. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) has been considering cell or animal models to be adopted to estimate the possible human health impacts after such exposure. The medaka fish is a good animal model for human-related health assessment studies; this paper examines both the embryo development and behavioral responses in medaka fish in vivo to long-term SMF exposure at the mT level. SMF exposure was examined for the complete developmental period of embryos until hatched; the embryos were monitored and recorded every 24 h for different morphological abnormalities in their developmental stages. The behavioral response of adult fish was also examined by analyzing their swimming velocities and positioning as compared with that of the control group. It was observed that there were no impacts on embryo development under prolonged exposure up to about 100 mT while the swimming behavior of the adult fish under exposure was different to the control group-the swimming movement of the treated group was more static, with an average velocity of 24.6% less as observed over a 24-h duration.

Radiobiological characteristics of descendant progeny of fish and amphibian cells that survive the initial ionizing radiation dose

PURPOSE

To evaluate the development of delayed lethal mutations, the production of medium borne lethal bystander signals, and the acquirement of radiosensitive or radioresistant traits in distant descendant progeny of fish and amphibian cells surviving ionizing radiation MATERIALS AND METHODS: American eel brain endothelial cells (eelB) and African clawed frog epithelial cells (A6) were initially irradiated with gamma rays at 0.5 Gy or 2 Gy. Ionizing radiation (IR)-surviving cells were grown for 27 population doublings (PDs) for eelB and 43 PDs for A6. Reproductive cell death as quantified by clonogenic survival assays was used to determine the development of delayed lethal mutations, the production of medium borne lethal bystander signals, and the acquirement of radiosensitive or radioresistant traits in the progeny survivors.

RESULTS

Only medium borne bystander signals produced by 2-Gy-irradiated eelB progeny survivors at 12 PDs could reduce the clonogenic survival of the bystander reporter cells. IR-induced delayed lethal mutations occurred in irradiated eelB cells at 15-18 PDs; however, subsequently propagated progeny cells retained normal replicative abilities. No IR-induced delayed lethal mutations developed in progeny of irradiated A6 cells at up to 43 PDs. eelB progeny survivors did not develop new radiosensitive or radioresistant traits while A6 progeny survivors acquired a new radiosensitive characteristic.

CONCLUSION

This study enriches the current literature on the radiobiological characteristics of distant surviving progeny of irradiated fish and amphibian cells and highlights cell-type/species-dependent differential responses to IR. This study is the first to examine the potential transgenerational effects of progenitor irradiation in amphibian cells.

Assessment of high-fat-diet-induced fatty liver in medaka

Fatty liver, which has been continuously becoming more common in a number of patients, is the most common liver disease. For detailed analysis, a useful model for fatty liver is needed and fish are considered as a potential candidate. We assessed through direct observation of the liver, which is the most conventional method for non-invasive analysis of progression in fatty liver. By using transparent medaka (Oryzias latipes), we were able to observe changes in fat deposition in the liver. An analysis of the progression of fatty liver using ultrasound showed a significant increase in echo intensity, which indicates that this is a useful examination method. In addition, we clarified a metabolite profile in the medaka liver fed a high-fat diet (HFD), which had not previously been shown in detail. This medaka model, allowing non-invasive and repetitive assessment, is a useful model for the analysis of diseases that cause fatty liver in which changes in detailed metabolites are identified.

Summary: Our medaka model allows for non-invasive and repetitive assessment and is useful in the analysis of fatty liver in which changes in detailed metabolites are identified.

Transgenerational DNA Methylation Changes in Daphnia magna Exposed to Chronic γ Irradiation

Our aim was to investigate epigenetic changes in Daphnia magna after a 25-day chronic external γ irradiation (generation F0 exposed to 6.5 μGy·h^-1 or 41.3 mGy·h^-1) and their potential inheritance by subsequent recovering generations, namely, F2 (exposed as germline cells in F1 embryos) and F3 (the first truly unexposed generation). Effects on survival, growth, and reproduction were observed and DNA was extracted for whole-genome bisulfite sequencing in all generations. Results showed effects on reproduction in F0 but no effect in the subsequent generations F1, F2, and F3. In contrast, we observed significant methylation changes at specific CpG positions in every generation independent of dose rate, with a majority of hypomethylation. Some of these changes were shared between dose rates and between generations. Associated gene functions included gene families and genes that were previously shown to play roles during exposure to ionizing radiation. Common methylation changes detected between generations F2 and F3 clearly showed that epigenetic modifications can be transmitted to unexposed generations, most likely through the germline, with potential implications for environmental risk.

Single Low-Dose Ionizing Radiation Induces Genotoxicity in Adult Zebrafish and its Non-Irradiated Progeny

This study investigated to what extent a single exposure to low doses of ionizing radiation can induce genotoxic damage in irradiated adult zebrafish (Danio rerio) and its non-irradiated F1 progeny. Four groups of adult zebrafish were irradiated with a single dose of X-rays at 0 (control), 100, 500 and 1000 mGy, respectively, and couples of each group were allowed to reproduce following irradiation. Blood of parental fish and whole-body offspring were analysed by the comet assay for detection of DNA damage. The level of DNA damage in irradiated parental fish increased in a radiation dose-dependent manner at day 1 post-irradiation, but returned to the control level thereafter. The level of DNA damage in the progeny was directly correlated with the parental irradiation dose. Results highlight the genotoxic risk of a single exposure to low-dose ionizing radiation in irradiated individuals and also in its non-irradiated progeny.

DNA alterations and effects on growth and reproduction in Daphnia magna during chronic exposure to gamma radiation over three successive generations

This study examined chronic effects of external Cs-137 gamma radiation on Daphnia magna exposed over three successive generations (F0, F1 and F2) to environmentally relevant dose rates (ranging from 0.007 to 35.4 mGy h(-1)). Investigated endpoints included survival, growth, reproduction and DNA alterations quantified using random-amplified polymorphic DNA polymerase chain reaction (RAPD-PCR). Results demonstrated that radiation effects on survival, growth and reproduction increased in severity from generation F0 to generation F2. Mortality after 21 days at 35.4 mGy h(-1) increased from 20% in F0 to 30% in F2. Growth was affected by a slight reduction in maximum length at 35.4 mGy h(-1) in F0 and by reductions of 5 and 13% in growth rate, respectively, at 4.70 and 35.4 mGy h(-1) in F2. Reproduction was affected by a reduction of 19% in 21 day-fecundity at 35.4 mGy h(-1) in F0 and by a delay of 1.9 days in brood release as low as 0.070 mGy h(-1) in F2. In parallel, DNA alterations became significant at decreasing dose rates over the course of F0 (from 4.70 mGy h(-1) at hatching to 0.007 mGy h(-1) after ∼21 days) and from F0 to F2 (0.070 mGy h(-1) at hatching to 0.007 mGy h(-1) after ∼21 days), demonstrating their rapid accumulation in F0 daphnids and their transmission to offspring generations. Transiently more efficient DNA repair leading to some recovery at the organism level was suggested in F1, with no effect on survival, a slight reduction of 12% in 21 day-fecundity at 35.4 mGy h(-1) and DNA alterations significant at highest dose rates only. The study improved our understanding of long term responses to low doses of radiation at the molecular and organismic levels in a non-human species for a better radioprotection of aquatic ecosystems.

Dose-dependent hepatic transcriptional responses in Atlantic salmon (Salmo salar) exposed to sublethal doses of gamma radiation

Due to the production of free radicals, gamma radiation may pose a hazard to living organisms. The high-dose radiation effects have been extensively studied, whereas the ecotoxicity data on low-dose gamma radiation is still limited. The present study was therefore performed using Atlantic salmon (Salmo salar) to characterize effects of low-dose (15, 70 and 280 mGy) gamma radiation after short-term (48h) exposure. Global transcriptional changes were studied using a combination of high-density oligonucleotide microarrays and quantitative real-time reverse transcription polymerase chain reaction (qPCR). Differentially expressed genes (DEGs; in this article the phrase gene expression is taken as a synonym of gene transcription, although it is acknowledged that gene expression can also be regulated, e.g., at protein stability and translational level) were determined and linked to their biological meanings predicted using both Gene Ontology (GO) and mammalian ortholog-based functional analyses. The plasma glucose level was also measured as a general stress biomarker at the organism level. Results from the microarray analysis revealed a dose-dependent pattern of global transcriptional responses, with 222, 495 and 909 DEGs regulated by 15, 70 and 280 mGy gamma radiation, respectively. Among these DEGs, only 34 were commonly regulated by all radiation doses, whereas the majority of differences were dose-specific. No GO functions were identified at low or medium doses, but repression of DEGs associated with GO functions such as DNA replication, cell cycle regulation and response to reactive oxygen species (ROS) were observed after 280mGy gamma exposure. Ortholog-based toxicity pathway analysis further showed that 15mGy radiation affected DEGs associated with cellular signaling and immune response; 70mGy radiation affected cell cycle regulation and DNA damage repair, cellular energy production; and 280mGy radiation affected pathways related to cell cycle regulation and DNA repair, mitochondrial dysfunction and immune functions. Twelve genes representative of key pathways found in this study were verified by qPCR. Potential common MoAs of low-dose gamma radiation may include induction of oxidative stress, DNA damage and disturbance of oxidative phosphorylation (OXPHOS). Although common MoAs were proposed, a number of DEGs and pathways were still found to be dose-specific, potentially indicating multiple mechanisms of action (MOAs) of low-dose gamma radiation in fish. In addition, plasma glucose displayed an apparent increase with increasing radiation doses, although the results were not significantly different from the control. These findings suggested that sublethal doses of gamma radiation may cause dose-dependent transcriptional changes in the liver of Atlantic salmon after short-term exposure. The current study predicted multiple MoA for gamma radiation and may aid future impact assessment of environmental radioactivity in fish.

Evidence relevant to untargeted and transgenerational effects in the offspring of irradiated parents

In this article we review health effects in offspring of human populations exposed as a result of radiotherapy and some groups exposed to chemotherapy. We also assess risks in offspring of other radiation-exposed groups, in particular those of the Japanese atomic bomb survivors and occupationally and environmentally exposed groups. Experimental findings are also briefly surveyed. Animal and cellular studies tend to suggest that the irradiation of males, at least at high doses (mostly 1Gy and above), can lead to observable effects (including both genetic and epigenetic) in the somatic cells of their offspring over several generations that are not attributable to the inheritance of a simple mutation through the parental germline. However, studies of disease in the offspring of irradiated humans have not identified any effects on health. The available evidence therefore suggests that human health has not been significantly affected by transgenerational effects of radiation. It is possible that transgenerational effects are restricted to relatively short times post-exposure and in humans conception at short times after exposure is likely to be rare. Further research that may help resolve the apparent discrepancies between cellular/animal studies and studies of human health are outlined.

Effects of X-ray irradiation of female mice in preconceptive period on polymorphism of simple repeats in DNA of offspring of different sex

Sibs groups of F1-offspring born by non-irradiated mice and by female mice exposed to X-ray radiation in preconceptive period (50-200 cGy) were compared. Arbitrary primed PCR revealed significantly increased polymorphism of simple DNA repeats in somatic tissues of the offspring from female mice irradiated in a dose of 200 cGy. The increase in DNA polymorphism in postmitotic brain tissues and in peripheral blood was more pronounced than in proliferating spleen tissues and in the epithelium of tail tip. In the tissues of female offspring from irradiated mothers, higher increase in DNA polymorphism was observed in comparison with the tissues of male offspring from the same mothers.

Genotoxic and reprotoxic effects of tritium and external gamma irradiation on aquatic animals

Aquatic ecosystems are chronically exposed to natural radioactivity or to artificial radionuclides released by human activities (e.g., nuclear medicine and biology,nuclear industry, military applications). Should the nuclear industry expand in the future, radioactive environmental releases, under normal operating conditions or accidental ones, are expected to increase, which raises public concerns about possible consequences on the environment and human health. Radionuclide exposures may drive macromolecule alterations, and among macromolecules DNA is the major target for ionizing radiations. DNA damage, if not correctly repaired, may induce mutations, teratogenesis, and reproductive effects. As such, damage at the molecular level may have consequences at the population level. In this review, we present an overview of the literature dealing with the effects of radionuclides on DNA, development, and reproduction of aquatic organisms. The review focuses on the main radionuclides that are released by nuclear power plants under normal operating conditions, γ emitters and tritium. Additionally, we fitted nonlinear curves to the dose-response data provided in the reviewed publications and manuscripts, and thus obtained endpoints commonly associated with ecotoxicological studies, such as the EDR(10). These were then used as a common metric for comparing the values and data published in the literature.The effects of tritium on aquatic organisms were reviewed for dose rates that ranged from 29 nGy/day to 29 Gy/day. Although beta emission from tritium decay presents a rather special risk of damage to DNA, genotoxicity-induced by tritium has been scarcely studied. Most of the effects studied have related to reproduction and development. Species sensitivity and the form of tritium present are important factors that drive the ecotoxicity of tritium. We have concluded from this review that invertebrates are more sensitive to the effects of tritium than are vertebrates.Because several calculated EDR10 values are ten times lower than background levels of γ irradiation the results of some studies either markedly call into question the adequacy of the benchmark value of 0.24 mGy/day for aquatic ecosystems that was recommended by Garnier-Laplace et al. (2006), or the dose rate estimates made in the original research, from which our EDR(10) values were derived, were under estimated, or were inadequate. For γ irradiation, the effects of several different dose rates on aquatic organisms were reviewed, and these ranged from 1 mGy/day to 18 Gy/day. DNA damage from exposure to y irradiation was studied more often than for tritium, but the major part of the literature addressed effects on reproduction and development. These data sets support the benchmark value of 0.24 mGy/day, which is recommended to protect aquatic ecosystems. RBEs, that describe the relative effectiveness of different radiation types to produce the same biological effect, were calculated using the available datasets. These RBE values ranged from 0.06 to 14.9, depending on the biological effect studied, and they had a mean of 3.1 ± 3.7 (standard deviation). This value is similar to the RBE factors of 2-3 recommended by international organizations responsible for providing guidance on radiation safety. Many knowledge gaps remain relative to the biological effects produced from exposure to tritium and y emitters. Among these are: Dose calculations: this review highlights several EDR(10) values that are below the normal range of background radiation. One explanation for this result is that dose rates were underestimated from uncertainties linked to the heterogenous distribution of tritium in cells. Therefore, the reliability of the concept of average dose to organisms must be addressed. Mechanisms of DNA DBS repair: very few studies address the most deleterious form of DNA damage, which are DNA DBSs. Future studies should focus on identifying impaired DNA DBS repair pathways and kinetics, in combination with developmental and reproductive effects. The transmission of genetic damage to offspring, which is of primary concern in the human health arena. However, there has been little work undertaken to assess the potential risk from germ cell mutagens in aquatic organisms, although this is one of the means of extrapolating effects from subcellular levels to populations. Reproductive behavior that is linked to alterations of endocrine function. Despite the importance of reproduction for population dynamics, many key endpoints were scarcely addressed within this topic. Hence, there is, to our knowledge,only one study of courtship behavior in fish exposed to γ rays, while no studies of radionuclide effects on fish endocrine function exist. Recent technical advances in the field of endocrine disrupters can be used to assess the direct or indirect effects of radionuclides on endocrine function. Identifying whether resistance to radiation effects in the field result from adaptation or acclimation mechanisms. Organisms may develop resistance to the toxic effects of high concentrations of radionuclides. Adaptation occurs at the population level by genetic selection for more resistant organisms. To date, very few field studies exist in which adaptation has been addressed, despite the fact that it represents an unknown influence on observed biological responses.

The National BioResource Project Medaka (NBRP Medaka): an integrated bioresource for biological and biomedical sciences

Medaka (Oryzias latipes) is a small freshwater teleost fish that serves as a model vertebrate organism in various fields of biology including development, genetics, toxicology and evolution. The recent completion of the medaka genome sequencing project has promoted the use of medaka as a comparative and complementary material for research on other vertebrates such as zebrafish, sticklebacks, mice, and humans. The Japanese government has supported the development of Medaka Bioresources since 2002. The second term of the Medaka Bioresource Project started in 2007. The National Institute for Basic Biology and Niigata University were selected as the core organizations for this project. More than 400 strains including more than 300 spontaneous and induced mutants, 8 inbred lines, 21 transgenic lines, 20 medaka-related species and 66 wild stock lines of medaka are now being provided to the scientific community and educational non-profit organizations. In addition to these live fish, NBRP Medaka is also able to provide cDNA/EST clones such as full-length cDNA and BAC/fosmid clones covering 90% of the medaka genome. All these resources can be found on the NBRP Medaka website (http://shigen.lab.nig.ac.jp/medaka/), and users can order any resource using the shopping cart system. We believe these resources will facilitate the further use of medaka and help to promote new findings for this vertebrate species.

Isolated spermatozoa as indicators of mutations transmitted to progeny

Spermatozoa comprise a large and homogeneous population of cells that may serve as an alternative to resource-intensive assays of transmissible mutations based on progeny. To evaluate mutagenic responses in spermatozoa derived from germ cells exposed to a mutagen at different stages of spermatogenesis, we compared cII mutant frequencies (MFs) in spermatozoa collected from male lambda transgenic medaka exposed to ethylnitrosourea (ENU) as either post-meiotic or pre-meiotic germ cells. cII MFs in spermatozoa exposed to ENU as spermatogonial stem cells were induced significantly, 9-fold, compared to controls, whereas, cII MFs in spermatozoa exposed as spermatozoa/late spermatids were not elevated. To directly compare responses in spermatozoa with those in progeny, we analyzed cII MFs directly in spermatozoa and in the offspring produced from identical sperm samples of ENU-exposed males. cII MFs in isolated spermatozoa exposed to ENU as post-meiotic germ cells were not significantly elevated, whereas 11-30% of the progeny derived from the identically exposed germ cells exhibited significantly elevated cII MFs, approximately 2-fold to >130-fold, compared to controls. The contradictory responses between spermatozoa and progeny analyses can be attributed to induced pre-mutational lesions that remain intact in spermatozoa but were not detected as mutations. Progeny analyses, by contrast, revealed mutant individuals with elevated cII mutant frequencies because persistent DNA damage in the spermatozoa was fixed as mutations in cells of the early stage embryo. Spermatozoa exposed to a mutagen as spermatogonial stem cells can provide an efficient means to detect the portion of transmissible mutations that were fixed as mutations in spermatozoa. The caveat is that direct analyses of mutations in spermatozoa excludes the contribution of mutations that arise from post-fertilization processes in cells of early stage embryos, and therefore may underestimate the actual frequency of mutant offspring.

Dermal morphogenesis controls lateral line patterning during postembryonic development of teleost fish

The lateral line system displays highly divergent patterns in adult teleost fish. The mechanisms underlying this variability are poorly understood. Here, we demonstrate that the lateral line mechanoreceptor, the neuromast, gives rise to a series of accessory neuromasts by a serial budding process during postembryonic development in zebrafish. We also show that accessory neuromast formation is highly correlated to the development of underlying dermal structures such as bones and scales. Abnormalities in opercular bone morphogenesis, in endothelin 1-knockdown embryos, are accompanied by stereotypic errors in neuromast budding and positioning, further demonstrating the tight correlation between the patterning of neuromasts and of the underlying dermal bones. In medaka, where scales form between peridermis and opercular bones, the lateral line displays a scale-specific pattern which is never observed in zebrafish. These results strongly suggest a control of postembryonic neuromast patterns by underlying dermal structures. This dermal control may explain some aspects of the evolution of lateral line patterns.

Development of diverse lateral line patterns on the teleost caudal fin

The lateral line is composed of mechanoreceptors, the neuromasts, which are distributed over the body surfaces of fish. We examine the development of neuromast patterns on the caudal fins of medaka and zebrafish. In medaka, the terminal neuromast is established just prior to the caudal fin formation. The terminal neuromast subsequently gives rise to a cluster of accessory neuromasts. In zebrafish, the terminal neuromasts vary in terms of both number and position, and they achieve their final positions relative to the caudal fin structures through migration. Subsequently, they give rise to four lines of accessory neuromasts that extend along the caudal fin. We show that developmental processes similar to those observed in medaka and zebrafish may account for a large variety of patterns in other teleost species. These results establish terminal neuromast patterning as a new model for the study of the developmental mechanisms underlying diverse lateral line patterns.

Transgenic lambda medaka as a new model for germ cell mutagenesis

To address the need for improved approaches to study mutations transmitted to progeny from mutagen-exposed parents, we evaluated lambda transgenic medaka, a small fish that carries the cII mutation target gene, as a new model for germ cell mutagenesis. Mutations in the cII gene in progeny derived from ethyl-nitrosourea (ENU)-exposed males were readily detected. Frequencies of mutant offspring, proportions of mosaic or whole body mutant offspring, and mutational spectra differed according to germ cell stage exposed to ENU. Postmeiotic germ cells (spermatozoa/late spermatids) generated a higher frequency of mutant offspring (11%) compared to premeiotic germ cells (3.5%). Individuals with cII mutant frequencies (MF) elevated more than threefold above the spontaneous MF (3 x 10(-5)) in the range of 10(-4) to 10(-3) were mosaic mutant offspring, whereas those with MFs approaching 1 x 10(-2) were whole body mutant offspring. Mosaic mutant offspring comprised the majority of mutant offspring derived from postmeiotic germ cells, and unexpectedly, from spermatogonial stem cells. Mutational spectra comprised of two different mutations, but at identical sites were unusual and characteristic of delayed mutations, in which fixation of a second mutation was delayed following fertilization. Delayed mutations and prevalence of mosaic mutant offspring add to growing evidence that implicates germ cells in mediating processes postfertilization that contribute to genomic instability in progeny. This model provides an efficient and sensitive approach to assess germ cell mutations, expands opportunities to increase understanding of fundamental mechanisms of mutagenesis, and provides a means for improved assessment of potential genetic health risks.

Trans-generational radiation-induced chromosomal instability in the female enhances the action of chemical mutagens

Genomic instability can be produced by ionising radiation, so-called radiation-induced genomic instability, and chemical mutagens. Radiation-induced genomic instability occurs in both germinal and somatic cells and also in the offspring of irradiated individuals, and it is characterised by genetic changes including chromosomal rearrangements. The majority of studies of trans-generational, radiation-induced genomic instability have been described in the male germ line, whereas the authors who have chosen the female as a model are scarce. The aim of this work is to find out the radiation-induced effects in the foetal offspring of X-ray-treated female rats and, at the same time, the possible impact of this radiation-induced genomic instability on the action of a chemical mutagen. In order to achieve both goals, the quantity and quality of chromosomal damage were analysed. In order to detect trans-generational genomic instability, a total of 4806 metaphases from foetal tissues from the foetal offspring of X-irradiated female rats (5Gy, acute dose) were analysed. The study's results showed that there is radiation-induced genomic instability: the number of aberrant metaphases and the breaks per total metaphases studied increased and were found to be statistically significant (p < or = 0.05), with regard to the control group. In order to identify how this trans-generational, radiation-induced chromosomal instability could influence the chromosomal behaviour of the offspring of irradiated rat females in front of a chemical agent (aphidicolin), a total of 2481 metaphases were studied. The observed results showed that there is an enhancement of the action of the chemical agent: chromosomal breaks per aberrant metaphases show significant differences (p < or = 0.05) in the X-ray- and aphidicolin-treated group as regards the aphidicolin-treated group. In conclusion, our findings indicate that there is trans-generational, radiation-induced chromosomal instability in the foetal cells from X-ray-treated female rats and that this RIGI enhances the chromosomal damage caused by the chemical agent aphidicolin.

Assessing radiation-associated mutational risk to the germline: repetitive DNA sequences as mutational targets and biomarkers

This review assesses recent data on mutational risk to the germline after radiation exposure obtained by molecular analysis of tandemly repeated DNA loci (TRDLs): minisatellites in humans and expanded simple tandem repeats in mice. Some studies, particularly those including exposure to internal emitters, indicate that TRDL mutation can be used as a marker of human radiation exposure; most human studies, however, are negative. Although mouse studies have suggested that TRDL mutation analysis may be more widely applicable in biomonitoring, there are important differences between the structure of mouse and human TRDLs. Mutational mechanisms probably differ between the two species, and so care should be taken in predicting effects in humans from mouse data. In mice and humans, TRDL mutations are largely untargeted with only limited evidence of dose dependence. Transgenerational mutation has been observed in mice but not in humans, but the mechanisms driving such mutation transmission are unknown. Some minisatellite variants are associated with human diseases and may affect gene transcription, but causal relationships have not yet been established. It is concluded that at present the TRDL mutation data do not warrant a dramatic revision of germline or cancer risk estimates for radiation.

The offspring of irradiated parents, are they stable?

Mutation induction in directly exposed cells is currently regarded as the main component of the genetic risk of ionising radiation for humans. However, recent studies showing that exposure to ionising radiation results in elevated mutation rates detectable in the non-irradiated progeny of exposed cells challenge the existing paradigm in radiation biology. This review describes some recent data on radiation-induced genomic instability in vitro and mainly focuses on the in vivo phenomenon of transgenerational instability, where elevated mutation rates are detected in the non-exposed offspring of irradiated parents. The possible mechanisms and implications of transgenerational instability are also discussed.

Seminiferous cord formation and germ-cell programming: epigenetic transgenerational actions of endocrine disruptors

The molecular and cellular control of embryonic testis development was investigated through an analysis of the embryonic testis transcriptome to identify potential regulatory factors for male sex determination and testis morphogenesis. One critical factor identified is neurotropin 3 (NT3). At the onset of male sex determination, Sertoli cells initiate differentiation and express NT3 to act as a chemotactic factor for mesonephros cells to migrate and associate with Sertoli-germ cell aggregates to promote cord formation. Promoter analysis suggests that NT3 may be an initial downstream gene to SRY and helps promote testis morphogenesis. Endocrine disruptors were used to potentially interfere with embryonic testis development and further investigate this biological process. The estrogenic pesticide methoxychlor and antiandrogenic fungicide vinclozolin were used. Previous studies have shown that methoxychlor and vinclozolin both interfere with embryonic testis cord formation and cause increased spermatogenic cell apoptosis in the adult testis. Interestingly, transient in vivo exposure to endocrine disruptors at the time of male sex determination caused a transgenerational phenotype (F1-F4) of spermatogenic cell apoptosis and subfertility. This apparent epigenetic mechanism involves altered DNA methylation and permanent re-programming of the male germ-line. A series of genes with altered DNA methylation and imprinting are being identified. Observations reviewed demonstrate that a transient embryonic in utero exposure to an endocrine disruptor influences the embryonic testis transcriptome and through epigenetic effects (e.g., DNA methylation) results in abnormal germ-cell differentiation that subsequently influences adult spermatogenic capacity and male fertility, and that this phenotype is transgenerational through the germ-line. The novel observations of transgenerational epigenetic endocrine disruptor actions on male reproduction critically impact the potential hazards of these compounds as environmental toxins. The literature reviewed provides insight into the molecular and cellular control of embryonic testis development, male sex determination, and the programming of the male germ-line.

Genotoxicological studies in aquatic organisms: an overview

Substantial progress has been made in the lat two decades to evaluate the impact of physical and chemical genotoxins in aquatic organisms. This overview (a) summarises the major high lights in this stimulating area of research, (b) compares the developments in this field with the developments in mammalian genotoxicological studies, where appropriate, (c) introduces 18 different articles presented in this special issue of Mutation Research in the backdrop of main advances and , (d) hypothesises on future directions of research in this exciting field.